In a prior art PLC distributed control system, the memory means, the processing portion, etc. are multiplexed to build up the safety system based on the predetermined safety standards. The safety controller set forth in JP-A-2002-358106, and the like, for example, have been publicly known. Further, in factory automation, a plurality of machine facilities are controlled simultaneously. In some cases the distributed control system is constructed by connecting mutually distinct PLCs that control the machine facilities respectively. Further, in such a distributed control system, a demand for safety orientation is escalating and the reliability of the communication process between respective PLCs should be secured sufficiently by multiplexing, in parallel, the communication lines between respective PLCs, etc.
Further, there is an increasing need for an interlocking mechanism with high reliability in communication control that can hold the safety communication in such distributed control system and that can be constructed easily and effectively. Further, there is an increasing need for the distributed control system to be operated/managed effectively by using an expression format that can express the safety processing sequence in this system in an easy-to-understand manner.